- Add error mapping to list_rating_plans in engine.rs
- Add dead_code attributes to get/list_map/deactivate methods in rating_plans_repo and row_to_plan function
- Add list method to RatingPlansRepo returning Vec<RatingPlan>
- Add dead_code attribute to validate_imsi in helpers
- Add UsageBlocked and ConfigurationError variants to ChargingError with Display/IntoResponse implementations
- Add Default
- Add AppState struct to hold shared ChargingEngine instance
- Add CreditCheckRequest and CreditCheckResponse for credit validation
- Add DeductRequest for byte usage deduction
- Add AddCreditRequest for credit top-up operations
- Add BalanceResponse for IP address balance queries
- Add HealthResponse with default implementation for health check endpoint
- Derive Serialize/Deserialize traits for API request/response types